The field relates generally to use of multi-fuel combustion to power an engine of the compression-ignition type. More particularly, the field relates to a two-stroke cycle opposed-piston engine equipped for injection of a main charge of gaseous fuel into a cylinder during a compression stroke, before the pistons reach top center (TC), together with injection of a pilot charge of liquid fuel into a shaped combustion chamber to ignite the main charge.
For reasons of performance and cost it can be advantageous to incorporate more than one fuel into the combustion system of a high power density engine. For example, the operating costs of a compression-ignition engine may be reduced by use of a gaseous fuel (such as natural gas (NG)) to provide the main fuel charge and small quantities of a more reactive liquid fuel (such as diesel) to ignite the main fuel charge.
In a conventional compression-ignition (CI) engine a single piston is slidably disposed in a cylinder. The piston moves in the cylinder between a top center location where the crown of the piston is closest to the cylinder head, and a bottom center (BC) position where the crown is furthest from the cylinder head. Air introduced into the cylinder is compressed by the piston as it moves toward TC during its compression stroke. Compression of the air raises its temperature. Fuel is injected into the heated air at a time when the piston nears the top of its compression stroke. The elevated temperature of the compressed air causes autoignition of the fuel whereby the fuel self ignites and burns, releasing energy and driving the piston toward BC in a power stroke.
In a two-stroke cycle, opposed-piston engine built for compression ignition, two pistons are slidably disposed crown-to-crown in the bore of a cylinder having intake and exhaust ports near BC of each piston. During engine operation, the ports are controlled by movement of the pistons as they move in opposition in the bore, toward and away from each other, between their TC and BC positions. Air introduced into the cylinder is compressed by the pistons as they move toward their respective TC positions during a compression stroke. The engine typically has one or more liquid fuel injectors mounted to the cylinder at a location near the TC position of the piston crowns. The injected fuel mixes with the compressed air and the air/fuel mixture autoignites, driving the pistons away from each other in a power stroke toward their BC positions.
Commonly-assigned U.S. Pat. No. 7,270,108 describes a two-stroke cycle opposed-piston engine in which injection of a main charge of liquid fuel early in the compression stroke provides more complete evaporation of the fuel by the time the stroke is nearly complete. Injection of a pilot charge of fuel into the compressed air/fuel mixture near TC of the pistons activates ignition of the main charge. Control of the pilot injection enables precise timing of ignition of the main fuel charge.
It is desirable to promote turbulence in the bulk motion of charge air in the cylinder bore during the compression stroke in order to mix air with fuel in preparation for combustion. Elements of charge air motion that contribute to turbulence include swirl and squish flow. In some aspects, the end surfaces of the piston crowns are made with shapes and contours designed to interact with these elements in order to produce additional bulk motion components such as tumble. Relatedly, combustion chambers have been constructed to produce complex turbulence so as increase the homogeneity of the mixing process. See, for example, the combustion chamber constructions described in US 2011/0271932 and WO 2012/158756.
As an opposed-piston engine runs, a combustion chamber is formed once each cycle of engine operation when the pistons meet near the center of the cylinder. Typically, an injector, or more than one injector, is mounted to the cylinder at, or very near, TC of the pistons. As a result, the injector heads are located in close proximity to the combustion chamber formed between the crowns of the pistons when the pistons are at TC. To avoid any possible interference problems between the injector heads and the piston crowns, careful design planning is necessary. When multiple injectors, (perhaps more than two) are to be used, injector locations are critical, especially in view of constraints imposed by combustion chamber constructions. This problem may be compounded in an opposed-piston engine built for multi-fuel combustion if different types of fuel delivery devices are incorporated into the fuel delivery system.